Medical robotic systems such as those used in performing minimally invasive surgical procedures offer many benefits over traditional open surgery techniques, including less pain, shorter hospital stays, quicker return to normal activities, minimal scarring, reduced recovery time, and less injury to tissue. Consequently, demand for minimally invasive surgery using such medical robotic systems is strong and growing.
Examples of medical robotic systems include the daVinci® Surgical System and the daVinci® S™ Surgical System from Intuitive Surgical, Inc., of Sunnyvale, Calif. Each of these systems includes a surgeon's console, a patient-side cart, a high performance three-dimensional (“3-D”) vision system, and Intuitive Surgical's proprietary EndoWrist™ articulating instruments, which are modeled after the human wrist so that when added to the motions of the slave manipulator holding the surgical instrument, they allow at least a full six degrees of freedom of motion, which is comparable to or even greater than the natural motions of open surgery.
The daVinci® surgeon's console has a high-resolution stereoscopic video display with two progressive scan cathode ray tubes (“CRTs”). The system offers higher fidelity than polarization, shutter eyeglass, or other techniques. Each eye views a separate CRT presenting the left or right eye perspective, through an objective lens and a series of mirrors. The surgeon sits comfortably and looks into this display throughout surgery, making it an ideal place for the surgeon to display and manipulate 3-D intraoperative imagery.
The patient-side cart typically includes three or more slave manipulators for holding and manipulating medical devices such as surgical instruments and image capturing devices for performing and viewing a medical procedure at a surgical site within a patient. To manipulate these medical devices, the surgeon's console also includes master input devices which may be selectively associated with the medical devices (and the slave manipulators holding the medical devices) to manipulate them. Since the movements of the master input devices and their associated medical devices are scaled, this allows the surgeon to perform intricate medical procedures with greater ease than with conventional open surgery. It may also allow the surgeon to perform medical procedures that are not even feasible using conventional open surgery techniques.
During very fine movement of the instruments during a medical procedure, however, corresponding movement of mechanical elements (such as joints, gears, spools, and other driven elements) used in moving the instrument or the slave manipulator holding the instrument may require fine motion velocities that induce stick-slip behavior in their motion. Since such behavior at the mechanical element level may be translated to the movement of their corresponding surgical instruments, this behavior may significantly diminish the ability of the surgeon to efficiently perform any medical procedure requiring fine manipulation of tissue, surgical sutures, needles etc., such as for example during an anasthomosis. Possible root causes for this problem include the presence of static friction (stiction) and the associated increase of dynamic friction (Coulomb friction) for velocities in the fine motions range. Due to the non-linear nature of this problem, and the specific requirements of medical robotic surgery, simple gain adjustments and/or the addition of dithering to a conventional proportional-integral-derivative (“PID”) joint controller may not acceptably resolve it.